The butt cap of a tennis racquet is conventionally a flat-ended octahedral cup of black polyethylene secured to the end of the handle of the racquet by staples or by glue. The conventional butt cap comprises only one cavity, for slidably engaging the handle end of the racquet, and functions to prevent the hand of the player from slipping below the handle end and as a platform for the manufacturer's logo and a cover for the part of the handle not wrapped by the grip. Very little of the conventional butt cap extends beyond the hand of a player gripping the racquet. Few improvements have been made to the butt cap, although racquets have undergone a profound transformation in nearly all other features.
Conventionally, a tennis racquet is 68.6 cm in length, although the rules of the International Tennis Federation allow for a maximum length of 81.2 cm. Recently, the advantages of a longer than conventional racquet length have been recognized by tennis professionals. See Tennis, Vol. 31, No. 5, September 1995, pp. 32-36. The butt end of such extra-long racquets is flat because they have a conventional butt cap. The extra length of 2.5-5.1 cm is added to the racquet by increasing the length of the racquet head and/or racquet handle, not by increasing any dimension of the butt cap. No means for increasing the length of an existing racquet by means of a butt cap is presently known to the art.
In U.S. Pat. No. 5,058,902 to McCutchen (1991), I disclosed a static distal butt weight for a tennis racquet handle, for the purpose, among others, of extending the center of percussion, absorbing shock, and increasing the momentum of the racquet. In FIG. 6 of that patent, a butt cap for adding the static distal butt weight was described. The means for attachment for the butt cap were stated to be glue or adhesive tape. The structure was that of a conventional butt cap with an octagonal cavity for receiving a handle end of a racquet. There was no cavity in that butt cap, other than the cavity for receiving the handle end. The doorknob-shaped configuration of the butt cap therein described was not such as would permit a comfortable grip below the handle end because of the sharp annular bulge of the butt cap beyond the handle end.
U.S. Pat. No. 5,094,453 to Douglas et al. (1992) shows a tennis racquet with a solid butt weight attached to its handle end by means of an "acorn shaped weighted extension" (4) attachable to the end of the handle by welding or screwing it on (5:8). The extension comprises a "circular disk section" (5) as at the end of a baseball bat, i.e. a sharp radial projection to keep the hand from sliding off the handle end. Such a circular disk section would make impossible any comfortable grip beyond the handle end because the circular disk section, a sharp annular bulge in the structure of the extension, would be in the palm of the player. Indeed, the whole purpose of the circular disk section is that of the annular bulge at the handle end of a conventional baseball batt--to prevent the gripping hand from going below the handle end.
U.S. Pat. No. 5,069,452 to Chen (1991) teaches a conventional butt cap covered by a resilient layer to provide a soft grip.
U.S. Pat. No. 4,984,793 to Chen (1991) teaches an elongated rubber sheath over a conventional butt cap, the sheath extending up the handle with numerous recesses for containing small individual optional weights under the player's hand.
U.S. Pat. No. 4,907,810 to Whiteford (1990) teaches a butt cap configured for the purpose of preventing the racquet slipping from the player's hand and to anchor the grip covering. (6: 64-68). The wails defining the cavity for receiving the racquet handle are slotted.
Prior art of tennis racquets generally considers that low overall racquet weight, distributed mostly in the part of the racquet comprising the string mesh, i.e. the racquet head, is desirable. For example, the Wilson Hammer.TM. is a light, stiff racquet having a weight distribution similar to that of a hammer: a heavy head and very light handle. But the overall weight, even of the extra-long racquets, is only approximately 311 grams.
Weak players may consider the overall light racquet weight to be an advantage for them, but there is a tradeoff in power and control. Momentum is conserved in any collision, so the lighter racquet must be swung faster to achieve the same ball velocity, and a faster swing means less control. Although the lighter racquet is easier to swing, so that the increased velocity may make up for the reduced racquet mass, when it meets the ball there is a severe deceleration. In the impact with the ball, a light racquet must give up more velocity than a heavy racquet, and this sudden (appproximately 0.004 seconds) braking action, i.e. shock, certainly has no beneficial effect on the muscles and tendons of the player's elbow or on the accuracy of the shot.
Shifting the center of mass further toward the head increases the moment of inertia, making the racquet more difficult to accelerate on the serve and less responsive in volleying. Players generally praise the head-heavy racquet for ground strokes, but condemn it for volleys and serves. The head weight may be small, but it is far from the player's hand and acts on a long lever arm.
It is well known that the velocity of propagation of mechanical waves varies as the square root of the ratio of the elastic modulus of the material to the density of the material. A light, stiff racquet therefore transmits shock very quickly. Vibration sets up in the racquet as the disturbance runs up and down the shaft. Various damping devices, disposed between the hand of the player and the racquet head, have been marketed. No damping means disposed beyond the handle end is known, except for my prior patent, U.S. Pat. No. 5,058,902.
It is also known in physics that a wave rebounding from a heavy medium back into a light medium has a phase shift of .pi. radians, i.e. destructive interference, whereas a wave in a heavy medium (such as a racquet shaft) rebounds from a relatively light medium (air) with no phase shift, i.e. constructive interference, so a standing wave sets up. The prior art of tennis racquets focuses on damping vibration once it is created, rather than preventing it.
Tennis elbow, also known as lateral epicondylitis, is a painful condition suffered by many tennis players. There is a great need for a solution to this problem, which appears to have increased during the recent vogue for light, stiff racquets. Vibration damping by means of padded grips, string attachments, asymmetric racquet heads, and assemblies within the racquet handle are the approaches that have been tried.
A stiff, light racquet tends to rotate backward on impact because of the moment contributed by the ball-racquet couple, spoiling the aim of the player, particularly on overheads. Moving the center of mass of the racquet farther from the hand and closer to the center of the strings decreases this moment, but increases the lever arm of the racquet mass center. A head-heavy weight distribution causes a sluggish feel and may, after ball release, contribute to tennis elbow by the following mechanism:
To overcome the moment contributed by the ball-racquet couple during the period of impact (approximately 0.004 seconds), the player must add a counterrotational arresting force by muscle alone, without aid of any ballast in the butt at all (in the conventional racquet). After the ball is released, the equilibrium of moments thus established by muscle force is suddenly disturbed, and the racquet head lurches forward, suddenly pulling on the muscles that attach to the elbow tendons. Then a distant mass center becomes a disadvantage because its longer lever arm gives it a greater pulling force on the elbow muscles and makes the lurch harder to arrest. In addition, the large shock from deceleration of the light racquet on impact causes the racquet to vibrate at the time the player is resisting this lurch.
Solutions of prior art for damping vibration in a racquet, such as padded grips, flexible racquet shafts, and dampers applied to the racquet strings, do not address the fundamental difficulty caused by small overall mass and head-heaviness. A more comprehensive solution is provided by the present invention, a butt cap which not only damps shock but prevents it by adding more mass where it does the most good, at the butt.